1. Description of the Invention
An object of the present invention is a contrast-setting or contrast-adjusting method that can be used, for example, during a contrast-reversal operation to go from an image displayed in a first mode, called a direct mode, to an image displayed in a second mode, called a reverse mode, or again to modify the contrast of a basic image. The invention is particularly applicable in the medical field where it can be used to compare radioscopic type images with radiographic type images. Nevertheless, it can be applied in other fields where, for example, for reasons of visual comfort, it is preferred to go from a direct image to a reverse image and vice versa.
In medicine, when a patient is subjected to X-radiation, there are two main ways of acquiring an image. A first method consists in placing a radiographic film on the other side of the patient with respect to the X-ray emitter. The radiosensitive layer of the film is imprinted by the radiation in varying degrees as a function of the radiological density of the parts crossed in the interposed body. After photographic development, the photograph is observed by transparency, for example by being placed against a luminous board. The image acquired under these conditions is called an image in radiographic mode. Another method consists in using a camera, or a detector of another type, to acquire an electronic image of the X-radiation that goes through the body. To this end, a target of a camera is placed after the body with respect to the X-ray emitter, and the target is read with electronic radiation. In practice, a scintillator screen is interposed between the body and the target of the camera. This scintillator screen converts the X-rays into light rays for whose passband the target is particularly sensitive. The video image thus acquired, called a radioscopic image, may be projected on the display monitor.
2. Description of the Prior Art
Owing to the different mode of acquisition of the direct image and of the image obtained by radioscopy, the contrasts of these images are reversed for one and the same examined structure. This means that the dark parts in one image are bright in the other one and vice versa. The practitioners are therefore used to making alternate examinations of an image of one type and an image of another type. It is therefore important to propose a system that enables the changing, for one and the same structure, from an image of one type to an image of another type because the practitioner may wish to look at it in a mode different from its mode of acquisition.
The images are processed notably to eliminate noise, or to make contour corrections or other corrections. As regards the contrast, several types of processing must be distinguished. One of the simplest corrections permitted by these systems is obtained by the presence of contrast-setting buttons located on the front face of the display monitors. It is possible, by acting on the luminosity and contrast buttons, to modify the appearance of the displayed image. Another correction, better known as a gamma correction, is a display correction related, firstly, to laws of vision (proper to the phenomenon of vision in the human eye) and secondly to phenomena of display (proper to the display screens used). There also exist contrast corrections that tend to increase the contrast in particular windows of contrast or even in geographic windows of the image.
The invention also relates to these latter types of processing. They could also be applied to the reverse images produced. The invention also relates to the modification or reversal of contrast. For monochromatic images (for color images, the problem is a threefold one), each image element or pixel may be assigned a value of luminosity on the screen ranging from a zero value to a maximum value. When the images are digitized, there is a known way of distributing the dynamic range of luminosity over a number 2.sup.N of levels (for example 256 levels ranging from 0 to 255). The number N of bits on which an image signal is coded at an image element represents this dynamic range.
During a contrast reversal, the image elements for which the luminosity is zero are assigned the maximum luminosity level, and the elements for which the luminosity levels were the maximum are assigned a zero luminosity. The elements for which the luminosity is intermediate are assigned an intermediate luminosity. The main problem to be resolved is that of choosing a good reversal function for the intermediate luminosities. For the contrast modification, also known as windowing, it is necessary to accentuate the contrast in a window of luminosity to the detriment of the contrast in the other parts.
Two functions of reversal can be envisaged in the prior art. If d designates the signal computed for an image element in direct mode, the signal i of the reverse image could be of the 2.sup.N-d (arithmetical reversal) or of the 2.sup.N /(d+1)-1 (geometrical reversal) type. However, these approaches do not truly correspond to images that can be easily interpreted. These approaches do not provide perceptible information to the practitioner's mind.
Rather than choosing simplistic functions, attempts have been made to use conversion functions of the following type: ##EQU1##
This group of reversal functions has been used to obtain reversed image contrasts that are desired and not imposed, without the possibility of making any changes therein. The possibility of choosing a value of A between 0 and 99 and B between 0.1 and 5 led to a large set of functions. The drawback of this method which, as it happened, appeared to be a simple one, was that practitioners did not truly have the means of understanding what they were doing in modifying the values A and B on which they were able to play. Initially, they were given systems of reversal curves corresponding to the parameters A and B. In practice, these systems of curves could hardly be understood and a plotting did not enable a sufficiently user-friendly interpretation of the coefficients A and B.
In the invention, it being known that it is possible to choose any reversal function between the minimum and the maximum (provided that this function is at least monotonic), it has been observed that ultimately the practitioners'desire was to set the contrast in the dark zones and the contrast in the light zones independently of each other. The idea of the invention then was to reverse the old ways and to ask practitioners to choose their contrast in the dark zones and in the clear zones, and then to compute the function of modification or reversal that corresponded to this choice. In other words, according to the invention, the practitioners have two commands at their disposal enabling these contrasts to be set. These settings then correspond to modifications of the appearance of the images that the human brain is capable of understanding. It is enough, for example, for a constant setting of contrast for the luminous zones, to modify the contrast of the dark zones to get an immediate view of what effect it has on the reversed image in the dark zones. Then, the best setting is chosen. Then a similar setting is made for the dark zones that become luminous by reversal. The particular feature of the invention, then, is that there is a perceptible monotonic correspondence between the action on one of the setting buttons and on the phenomenon displayed on the screen.